1. Field of the Invention
The present invention relates to stabilizer systems for curable thiol-ene formulations. More particularly, the invention relates to a stabilizer system comprising an alkenyl substituted phenolic compound in combination with one or more of the following compounds: a free radical scavenger, a hindered phenolic antioxidant, or a hydroxylamine derivative.
2. Description of the Related Art
The synthesis of polythioethers by crosslinking polyenes with polythiols is well-known in the art. These compositions can be cured by photopolymerization initiated by actinic radiation, in particular radiation in the UV-visible range, or by the use of high energy electron beam radiation or by exposure of the reactants to chemical free radical generating agents.
A preferred group of thiol-ene compositions are those in which the ene is a bicyclic compound such as norbornene as described in U.S. Pat. No. 4,808,638. These thiol-nene compositions are useful in a variety of applications because of their rapid photocure speeds and the wide variety of cured product profiles available to the formulator.
Though thiol-nene compositions have many advantages, it is difficult to stabilize them, especially to attain long term shelf stability. Formulations which have long term shelf stability are defined as those which do not double their viscosity in six months at room temperature. An increase in viscosity indicates that a spontaneous oligomerization reaction is occuring between the polyene and the polythiol. In extreme cases, the unstable formulations will result in intractable, insoluble gels.
Conventional stabilizers for thiol-ene formulations include hydroquinone, p-tert butyl catechol, 2,6-ditert-butyl-p-methylphenol, phenothizaine, N-phenyl-2-naphthylamine, inert gas atmospheres such as helium, argon, nitrogen, carbon dioxide and acidic stabilizers such as phosphorous acid as disclosed in U.S. Pat. No. 3,787,303. Additional stabilizers include the free radical scavenger vinyl stabilizers disclosed in U.S. Pat. No. 3,619,393 and the metal complexes of N-nitrosophenylhydroxylamine disclosed in U.S. Pat. No. 4,985,471. However, these conventional stabilizers are not effective in thiol-nene formulations. This is probably because norbornenyl groups are almost an order of magnitude more reactive with polythiols than are enes such as butyl vinyl ether as disclosed in Radiation Curing of Polymeric Materials, Hoyle & Kinstle eds., ACS Symposium Series 417, pgs 165-167.
Several references have proposed stabilizers specifically for thiol-nene formulations. For example, EPO Application No. 0428342A discloses that certain N-nitrosohydroxyl amines and salts thereof function as stabilizers for radically curable norbornenyl-thiol formulations. While this provides a marked improvement over conventional stabilizers and allows for the production of commercially acceptable short-term stable products, it does not provide consistent and sufficient long-term stability necessary for the formulations to have a broad commercially acceptable shelf life and also is very sensitive to trace impurities in the nene resin. U.S. Pat. No. 5,208,281 discloses a stabilization system for thiol-nene formulations comprising N-nitrosoarylhydroxylamine and an active iodine species. While this stabilizer system provides a better stability than conventional stabilizers and is also less sensitive to trace impurities in the nene resin, it does not provide the desired long-term stability.
Therefore, there is still a need for an improved stabilizer system for thiol-ene and particularly thiol-norbornenyl (thiol-nene) formulations which provides long-term shelf stability without significantly adversely impacting cure speed.